High purity nitrogen at medium pressures within the range of from 40 to 95 pounds per square inch absolute (psia) is used for many purposes such as blanketing, stirring, conveying, pressurizing, inerting and purging in many industries such as in the electronics, glass, aluminum and chemical industries. Generally such nitrogen is produced in a single column air separation plant wherein nitrogen is the only product. In some situations it would be desirable to produce commercially usable oxygen along with the nitrogen, for example for use in oxygen or oxygen-enriched air combustion.
The production of oxygen and nitrogen by cryogenic air separation has long been known by use of a double column plant wherein a larger lower pressure column is in heat exchange relation with a smaller higher pressure column. Unfortunately such a conventional double column produces nitrogen at only a few pounds per square inch greater than atmospheric pressure. This necessitates costly compression of the nitrogen to achieve the desired higher pressure.
There are known cryogenic air separation methods which can produce medium pressure nitrogen and small amounts of very high purity oxygen. Such methods are disclosed in U.S. Pat. No. 4,560,397--Cheung and U.S. Pat. No. 4,783,210--Ayres et al. However, such methods can produce only a small amount of oxygen and thus their utility is limited when significant quantities of commercially usable oxygen are required.
Accordingly it is an object of this invention to provide a cryogenic air separation method which can produce nitrogen at elevated pressure and can also produce significant quantities of commercially usable oxygen.